1. Field of the Invention
The present invention relates to a process, and also to an apparatus, for improved hydrolytic cleavage of carboxylic esters to the corresponding carboxylic acid and alcohol, in which at least a portion of the water required for hydrolysis is replaced by a recycled mixture comprising carboxylic acid and water which results from the hydrolysis of carboxylic ester already converted in the process.
2. Description of the Related Art
The inverse of esterification is the hydrolysis of the carboxylic ester to carboxylic acid and alcohol. Like the esterification itself, the acid-catalyzed hydrolysis is an equilibrium reaction. Carboxylic esters are used in the chemical industry, for example, as solvents or plasticizers, or are obtained in various reactions as by-products or main products. For example, methyl acetate is a typical by-product in the preparation of polyvinyl alcohol. Mixtures comprising methyl acetate which are obtained in the preparation of polyvinyl alcohol, in addition to a small amount of low-boiling substances, for example acetaldehyde, comprise an azeotropic mixture of methyl acetate and methanol. Mixtures of the composition described can be economically utilized only with difficulty, if at all, without further purification or cleavage (hydrolysis) of the carboxylic esters to the reactants.
The hydrolysis of carboxylic esters can be carried out batchwise or continuously. Suitable for this purpose is a reactor in combination with a conventional distillation or a reactive distillation column. The use of a reactor in combination with a distillation column for the hydrolysis of methyl acetate is described, for example, in the patent U.S. Pat. No. 4,352,940. The process described there for hydrolyzing methyl acetate is disadvantageous, among other reasons, because the yields are small and a plurality of distillation stages is necessary. In addition, corrosion occurs as a result of the use of sulfuric acid or hydrochloric acid as homogeneous catalysts.
The patent specification U.S. Pat. No. 5,113,015 describes a process for hydrolyzing methyl acetate, in which methyl acetate and water are contacted with a catalyst packing in a distillation column, which hydrolyzes methyl acetate to acetic acid and methanol and at the same time at least partially separates the reaction mixture in the separating column.
The patent specification U.S. Pat. No. 5,770,770 likewise discloses a process for hydrolyzing a methyl acetate mixture in a reactive distillation column, in which the hydrolysis of a methyl acetate stream containing at least 50% by weight of methyl acetate takes place in a reaction zone having an ion exchanger packing. The methyl acetate mixture and water are conducted in countercurrent over the ion exchanger packing. Unconverted methyl acetate and water are recycled into the reaction zone. The hydrolysis products are removed from the bottom fraction. The impurities are recycled back into the reaction zone.
The teachings of the patents U.S. Pat. No. 5,113,015 and U.S. Pat. No. 5,770,770 is the use of strongly acidic ion exchangers as catalysts for the hydrolysis of carboxylic esters. The patent U.S. Pat. No. 5,770,770 further describes the use of the ion exchanger material preferably in the form of Raschig rings, whereas, in U.S. Pat. No. 5,113,015, the catalyst material is preferably used in the form of mats. The catalyst packing material used may be pliable, open-meshed substances, for example wire mesh, rigid, cellular monoliths of steel, polymers or ceramic material, and also corrugated metal, plastics or ceramic plates.
The last few processes described which use only a single reactive distillation column have the disadvantage that methyl acetate is only partly hydrolyzed. The reactant stream makes necessary the use of at least one additional purification stage. It is also disadvantageous that metal ions from the methyl acetate feed deactivate the catalyst in the reactive distillation column, which leads to short on-stream times of such continuous plants.
The European laid-open specification EP 1 220 825 A2 describes a process in which the feed stream comprising the carboxylic ester is fed into a prereactor in which the carboxylic ester is contacted with a first catalyst in the presence of water, which partly hydrolyzes the carboxylic ester to the hydrolysis products. The reaction mixture from the prereactor is passed into a reactive distillation column and is contacted with a second hydrolysis catalyst for the purpose of at least partial conversion of the remaining carboxylic ester to the corresponding carboxylic acid and alcohol.
The process described in EP 1 220 825 has the advantage that a higher conversion rate can be attained than in the aforementioned processes. In addition, the use of a prereactor improves the on-stream time of the reactive distillation column, since catalyst poisons, for example metal ions, remain principally in the prereactor. The selection of the particle size of the catalyst material used in the prereactor is less restricted than for the structured catalyst packings used advantageously in the reactive distillation column.
Even when there is initial increased recycling of methyl acetate-rich top stream of the reactive distillation column to the prereactor, the capacity using an existing structure is limited as a consequence of the fixed geometry of the reaction part of the reactive distillation column.
The process described in EP 1 220 825 also has the disadvantage that, when the feed stream of carboxylic ester comprises the corresponding alcohol, the chemical equilibrium of the acid-catalyzed hydrolysis is shifted unfavourably for the products in the reaction part of the reactive distillation column.